Orthostatic Headache: This condition is characterized by headaches that worsen in upright positions and improve or resolve when lying down. It can be primary (without an identifiable cause) or secondary to other conditions such as dehydration, blood loss, or certain medications. The absence of findings on MRI supports this diagnosis, as orthostatic headaches are often related to changes in cerebrospinal fluid (CSF) pressure or volume rather than structural abnormalities.

Spontaneous Intracranial Hypotension (SIH): Although MRI might not always show the characteristic findings of SIH, such as meningeal enhancement or brain sagging, it remains a possible cause of positional headaches. CSF leak can lead to decreased intracranial pressure, causing headaches that are typically worse when standing.
Cervicogenic Headache: These headaches are referred from the cervical spine and can be exacerbated by certain positions. While MRI of the brain might be negative, imaging of the cervical spine could reveal abnormalities contributing to the headache.
Migraine: Some migraine headaches can have a positional component, although this is less common. The absence of structural abnormalities on MRI does not rule out migraine, as it is a functional disorder.

Subarachnoid Hemorrhage (SAH) with Negative Initial Imaging: Although extremely rare, it's crucial to consider SAH, especially if the headache is described as "the worst headache of my life." Initial imaging might be negative, necessitating lumbar puncture for diagnosis.
Cerebral Venous Sinus Thrombosis: This condition can present with headache and might not always be evident on initial MRI, especially if the thrombosis is small or if the MRI protocol does not include venous phase imaging.
Pituitary Apoplexy: A medical emergency that can present with sudden, severe headache, often with visual disturbances. Initial MRI might not always capture the acute changes, but it's a diagnosis that cannot be missed due to its severe implications.

Chiari Malformation: While often associated with structural abnormalities visible on MRI, some cases might have subtle findings that are easily overlooked, or the malformation might not be significant enough to be detected on standard MRI sequences.
CSF Leak: Similar to SIH, but can occur without the typical MRI findings, especially if the leak is small or intermittent.
Vestibular Migraine: A subtype of migraine that can include positional vertigo and headache, which might not have a clear structural correlate on MRI.